Tomographic and Radiographic Pelvimetry in the Brown-throated Sloth (Bradypus variegatus): a Morphometric Study

Abstract

Background: The brown-throated sloth (Bradypus variegatus) is an arboreal xenarthran of Neotropical forests, exhibiting adaptations for suspensory locomotion. The pelvic girdle constitutes a fundamental structural element in mammals, serving as the primary interface for weight transfer between the axial skeleton and hindlimbs, contributing to locomotor mechanics. The pelvic morphology It is important to understand the birth outcomes, with the cephalopelvic disproportion representing a leading cause of dystocia in domestic and wildlife species. Pelvimetry, performed radiographically or tomographically, constitutes a technique that can be used to evaluate pelvic morphology, predict dystocia risk, and estimate biological parameters in domestic animals and species of wildlife. However, there are no published pelvimetric data for B. variegatus. This study was conducted to address this knowledge gap through pelvic tomographic and radiographic analysis of B. variegatus. The objectives were to morphometrically characterise the pelvic dimensions of the samples included in this study, classify pelvic typology, and investigate the correlations between pelvic dimensions and body size.

Materials, Methods & Results: Nine frozen cadavers of Bradypus variegatus obtained from routine admissions to the Veterinary Hospital were used. The samples were classified into age/weight categories according to established criteria: stillborn (n = 2), infant (n = 5) and juvenile (n = 2). The sex distribution was 6 males, 1 female, and 2 undetermined (stillborns). Before imaging, each specimen was weighed and the length of the body was measured from the cranio-cervical transition point to the last caudal vertebra. A juvenile male underwent pelvic CT imaging and 8 cadavers underwent radiographic examination in ventro-dorsal and dorso-ventral projections. Ten pelvic diameters were measured digitally: superior bi-iliac diameter (DBIS), distance between iliac crests (DCI), middle bi-iliac diameter (DBIM), inferior bi-iliac diameter (DBII), distance between pubic symphysis tubercles (DTSP), distance between ischiatic tuberosities (DTI), right diagonal diameter (DDD), left diagonal diameter (DDE), sacro-pubic diameter (DSP), and distance between ischiatic spines (DEI). The pelvic inlet area (AEP) was calculated as (DBIM/2) × (DSP/2) × π. Statistical analyses were performed using BioEstat software version 5.3 (Instituto Mamirauá, Amazonas, Brazil).

Discussion: This study provides the 1^st quantitative pelvimetric data set for Bradypus variegatus. The main limitations include the modest sample size (n = 9) characteristic of rare wildlife anatomy studies, the absence of adult specimens preventing the characterisation of pelvic morphology of adult animals and the evaluation of sexual dimorphism. Progressive increases in DBIS, DBII, and AEP can support evidence-based treatment in rehabilitation settings. The mesatipelvic conformation suggests that dystocia investigations should prioritise foetal factors, positioning abnormalities, and maternal parity, although confirmation requires studies that incorporate adult females and observations of pregnancy. The dual-modality approach demonstrates that CT provides superior anatomical detail, while radiography yields linear measurements. These findings may be fundamental veterinarians in captive breeding programmes for this threatened species facing habitat fragmentation and anthropogenic threats. Future research priorities include expanding this data set to include adults of both sexes, validating measurements in vivo, and integrating pelvimetric data with pelvic limb myology and biomechanical for later application in reproductive health evaluation and species conservation.

Keywords: common sloth. computed tomography. pelvic morphometry. pelvic radiography. xenarthran morphology.